1. Field of the Disclosure
This disclosure concerns optical wavelength filters having cascaded stages, the stages comprising birefringent retarders between polarizers. More particularly, the present disclosure relates to optical filters wherein polarizers that respectively precede and follow one or more of the retarder stacks are configured to include both absorptive and zero-degree reflective type polarizers.
2. Brief Description of Related Art
In a tunable birefringent filter, stacked retarders in a filter stage (or in plural cascaded stages) comprise fixed crystal retarders joined with tunable liquid crystals. The retarders are tuned in unison to adjust the wavelength transmission characteristic of the filter stage. Each filter stage has a comb transmission characteristic. With tuning, the transmission characteristics are adjustable such that bandpass peaks of successive stages overlap to discriminate for a selectable narrow wavelength band with high finesse.
A birefringent filter with multiple stages is disclosed, for example, in U.S. Pat. No. 6,992,809—Wang, the teachings of which are hereby incorporated by reference in their entireties. In a birefringent filter as described therein, orthogonal light components are differentially retarded by each retarder in a stack of retarders in each of several cascaded stages separated by polarizers. The differential retardation changes the polarization state of light passing through each respective retarder in a stack, causing different wavelengths to assume different polarization states. The polarization change caused by each member in the sequence of retarders in a stage is coordinated with the other members, so that selected wavelengths align with the exit polarizer and are passed to the next stage in cascade. The retarders are typically plates oriented normal to the axis of light propagation. The birefringences (typically thicknesses) and rotational angles of the retarders are arranged to achieve this effect.
There are several known configurations with stacked retarder thicknesses and related rotational angles that are useful in this way. Known configurations include (without limitation) the Solc, Lyot and Evans configurations, as well as some hybrid types. These configurations vary as to the number of polarizers used, the equal or unequal thicknesses of the retarders and the respective rotational angles of the retarders and the polarizers. At least one polarizer is used as a selection polarizer at the exit or output in each case.
The Solc, Lyot and Evans filter configurations were developed originally for spectral analysis of light in astrophysics, and typically consisted of one stage having several fixed crystals as the stacked birefringent retarders. In U.S. Pat. No. 6,992,809—Wang, cited above, plural cascaded stages of stacked retarders and interleaved polarizers are arranged along a light propagation path. One or more of the stages is tunable. Each retarder in a tunable stage comprises a liquid crystal controllable birefringence. For example, electrically tunable liquid crystals can be affixed to fixed birefringence crystal retarders. The fast and slow birefringence axes of the fixed and tunable birefringences are aligned. Tuning the liquid crystal, for example to increase birefringence, thus increases the retardation of the composite fixed and tunable elements, similar to what might be accomplished by making a fixed crystal thicker.
Birefringence filters are also known as interference filters. The transmitted wavelengths define a comb transmission characteristic, namely successive peaks at periodically related wavelengths, each peak having a given band width (generally measured as full width of a passband at half maximum amplitude, abbreviated “FWHM”). The transmission characteristic is periodic because a given retardation (whether considered as time or propagation distance) corresponds to the same phase angle for a succession of wavelengths. The distance between adjacent peaks is termed the free spectral range (“FSR”). It is desirable in a highly discriminating filter to have both a very narrow pass band width and a very large free spectral range. The ratio of these two factors is defined as the finesse of the filter. (Finesse=FSR/FWHM.)
In some possible configurations of Solc, Lyot, Evans and similar filters, the retardations contributed by each element in a stack of retarders is equal, typically by making the retarders equal in thickness along the light propagation path. In other configurations, the retarders have thicknesses that are related but not equal (e.g., thicknesses of “d,” “2d” and “d” in sequence). In a tunable embodiment as in U.S. Pat. No. 6,992,809—Wang, all the retarders in a stage are tuned in a coordinated way to maintain the same relationship. That is, if the retarders in a stack are equal according to the particular filter configuration (such as in Solc filters, for example), then all the liquid crystal elements are tuned to alter retardations equally for each member of the stack.
The effect of tuning the stage tends to expand or contract the comb filter transmission characteristic along the wavelength scale. Expanding the characteristic along the wavelength scale advantageously increases the free spectral range between peaks, but disadvantageously widens the pass band width. Conversely, tuning to contract the comb filter characteristic on the wavelength scale narrows the pass band width but disadvantageously reduces free spectral range.
The retarders within a stage are tuned in a coordinated way as necessary to expand or contract the comb shaped transmission characteristic on the wavelength scale, in order to place a pass band peak (or a band stop null) at a wavelength to which the filter is to be tuned. This is one way in which tuning is coordinated according to a technique as disclosed in Wang '809.
It is also an aspect of the technique in Wang '809 that successive filter stages are cascaded. In a band pass application (as opposed to band stop), cascading the transmission characteristics of two filters causes the transmission characteristics to multiply. Thus, at wavelengths where band pass peaks in the two filter characteristics coincide, multiplying the transmission characteristics advantageously causes the FWHM bandwidth of the cascaded filter peak to become narrower. At wavelengths where band pass peaks of one characteristic coincide with a low transmission null in the other characteristic, the resulting product is a null, which provides free spectral range between the next adjacent higher and lower bandpass peaks. If the stages of the filter have a given finesse value, cascading the stages produces a finesse equal to the product of the finesse values of the cascaded stages.
With several cascaded stages, preferably wherein the stages are tuned in a coordinated way, the cascaded filter has a high finesse and the ability to tune to bandpass wavelengths over a wide tuning range. However, each cascaded stage requires at least one polarizer at the exit of the propagation path through the sequence of rotationally oriented retarders of that stage. The exit polarizer is the selection element that passes light energy at wavelengths that the stacked retarders have presented in a polarization state aligned to the polarizer, and blocks light energy orthogonal to the polarizer. Each stage could also have an entrance polarizer to establish a starting polarization state, but typically, the exit or selection polarizer of a given stage functions as the entrance polarizer that establishes the polarization alignment of light propagating through to the next stage.
Each additional polarizer reduces the transmission ratio of light in the pass band to an extent. Polarizers block a high percentage of light energy aligned orthogonal to the polarizer, but also block a percentage of the light energy in the polarization state that is aligned parallel to the polarizer. It would be advantageous to employ a polarizer that has a high transmission ratio for light parallel to the polarizer and a high rejection ratio for light orthogonal to the polarizer. These aspects may be termed a high transmission ratio and a high degree of contrast.
Apart from ratio of transmission or rejection, there are additional considerations affecting the desirability of one kind or another. These considerations include the extent to which the polarizer acts on light that is normal to the plane of the polarizer versus off axis. Some types of polarizers maintain the direction of propagation and others divert orthogonal components. Some polarizers absorb relatively more of the rejected light energy and others reflect the rejected light energy, either opposite to the incident direction or otherwise.
An absorptive type polarizer can be produced as a stretched sheet of plastic film with a dichroic dye as a dopant. Dichroism is the property of some crystals and molecules to absorb light of one of two orthogonal polarization alignments more than light of the other alignment. Stretching the sheet or film orients the dichroic dopant molecules. The working principle is that for incident light containing components parallel to orthogonal polarization axes, one polarization is more strongly absorbed by the dye in the polymer film. The other polarization is more strongly transmitted. There is some loss in the transmitted component and some transmission of the orthogonal component. Although an absorptive sheet polarizer may fall at any point in a range of specifications, a typical transmission ratio is 60 to 93% (defined as the ratio of transmitted light energy when all the light energy is aligned to the transmission axis of the polarizer).
Another way to provide a polarizer is to provide a grid plate structure comprising parallel elements that are parallel and spaced so as to affect the extent to which light energy with orthogonal polarization components can propagate through. A so-called “wire grid” polarizer may have parallel wire-like lines at a given spacing or pitch (sometimes inaccurately termed the “period”). There are alternative ways to form the parallel linear elements that resemble wires. Therefore, another way to classify polarizers is wire-grid polarizers and non-wire-grid polarizers.
A polarizer may use effects other than dichroism and fenestrated linear grid structures. Certain crystalline structures and structures having interfaces of elements with distinct optical indices can be used as polarizers. Examples are the Brewster angle polarizer and the Glan-laser polarizer, which are both examples of non-wiregrid polarizers.
Accordingly, different sorts of polarizers can be classified according to various categories and subcategories. There are absorptive polarizers and reflective ones. Reflective polarizers can be zero-degree reflective polarizers (transmissive and reflective of orthogonal components that are strictly normal to the plane of polarizer plate) or non-zero degree reflective, for example diverting one orthogonal component relative to the other or perhaps reflective diffusely. In addition to these categories, polarizers might comprise thin sheets, surfaced plates, crystals with a substantial thickness or polarizing cubes.
These categorizations are partly structural and partly functional. For purposes of this disclosure, two important considerations are the extent to which the rejected orthogonal component of the light energy is absorbed (e.g., by dichroism) or diverted, and if diverted, whether the rejected component is directed precisely backwards along the propagation path.